a philosophical theory or approach that emphasizes the existence of the individual person as a free and responsible agent determining their own development through acts of the will. (

Existentialism is a term applied to the work of certain late 19th- and 20th-century philosophers who, despite profound doctrinal differences,shared the belief that philosophical thinking begins with the human subject—not merely the thinking subject, but the acting, feeling, living human individual. In existentialism, the individual's starting point is characterized by what has been called "the existential attitude", or a sense of disorientation and confusion in the face of an apparently meaningless or absurd world.Many existentialists have also regarded traditional systematic or academic philosophies, in both style and content, as too abstract and remote from concrete human experience. (Wikipedia)

Existence precedes essenceA central proposition of Existentialism is that existence precedes essence, which means that the most important consideration for individuals is that they are individuals—independently acting and responsible, conscious beings ("existence")—rather than what labels, roles, stereotypes, definitions, or other preconceived categories the individuals fit ("essence"). The actual life of the individuals is what constitutes what could be called their "true essence" instead of there being an arbitrarily attributed essence others use to define them. Thus, human beings, through their own consciousness, create their own values and determine a meaning to their life. Although it was Sartre who explicitly coined the phrase, similar notions can be found in the thought of existentialist philosophers such as ﻿Heidegger, and Kierkegaard:﻿

The Absur﻿d or AbsurdismThe notion of the Absurd contains the idea that there is no meaning in the world beyond what meaning we give it. This meaninglessness also encompasses the amorality or "unfairness" of the world. This contrasts with the notion that "bad things don't happen to good people"; to the world, metaphorically speaking, there is no such thing as a good person or a bad person; what happens happens, and it may just as well happen to a "good" person as to a "bad" person.

FacticityFacticity is a concept defined by Sartre in Being and Nothingness as the in-itself, of which humans are in the mode of not being. This can be more easily understood when considering it in relation to the temporal dimension of past: one's past is what one is, in the sense that it co-constitutes oneself. However, to say that one is only one's past would be to ignore a significant part of reality (the present and the future), while saying that one's past is only what one was, would entirely detach it from oneself now. A denial of one's own concrete past constitutes an inauthentic lifestyle, and the same goes for all other kinds of facticity (having a body—e.g. one that doesn't allow a person to run faster than the speed of sound—identity, values, etc.)Facticity is both a limitation and a condition of freedom. It is a limitation in that a large part of one's facticity consists of things one couldn't have chosen (birthplace, etc.), but a condition in the sense that one's values most likely depend on it. However, even though one's facticity is "set in stone" (as being past, for instance), it cannot determine a person: The value ascribed to one's facticity is still ascribed to it freely by that person. As an example, consider two men, one of whom has no memory of his past and the other remembers everything. They have both committed many crimes, but the first man, knowing nothing about this, leads a rather normal life while the second man, feeling trapped by his own past, continues a life of crime, blaming his own past for "trapping" him in this life. There is nothing essential about his committing crimes, but he ascribes this meaning to his past.However, to disregard one's facticity when, in the continual process of self-making, one projects oneself into the future, would be to put oneself in denial of oneself, and would thus be inauthentic. In other words, the origin of one's projection must still be one's facticity, though in the mode of not being it (essentially). Another aspect of facticity is that it entails angst, both in the sense that freedom "produces" angst when limited by facticity, and in the sense that the lack of the possibility of having facticity to "step in" for one to take responsibility for something one has done also produces angst.What is not implied in this account of existential freedom, however, is that one's values are not immutable; a consideration of one's values may cause one to reconsider and change them. A consequence of this fact is that one is responsible for not only one's actions, but also the values one holds. This entails that a reference to common values doesn't excuse the individual's actions. Even though these are the values of the society of which the individual is part, they are also their own in the sense that they could choose them to be different at any time. Thus, the focus on freedom in existentialism is related to the limits of the responsibility one bears as a result of one's freedom: the relationship between freedom and responsibility is one of interdependency, and a clarification of freedom also clarifies that for which one is responsible.

AuthenticityMany noted existentialist writers consider the theme of authentic existence important. Authentic existence involves the idea that one has to "create oneself" and then live in accordance with this self. What is meant by authenticity is that in acting, one should act as oneself, not as "one" acts or as "one's genes" or any other essence requires. The authentic act is one that is in accordance with one's freedom. Of course, as a condition of freedom is facticity, this includes one's facticity, but not to the degree that this facticity can in any way determine one's choices (in the sense that one could then blame one's background for making the choice one made). The role of facticity in relation to authenticity involves letting one's actual values come into play when one makes a choice (instead of, like Kierkegaard's Aesthete, "choosing" randomly), so that one also takes responsibility for the act instead of choosing either-or without allowing the options to have different values.In contrast to this, the inauthentic is the denial to live in accordance with one's freedom. This can take many forms, from pretending choices are meaningless or random, through convincing oneself that some form of determinism is true, to a sort of "mimicry" where one acts as "one should." How "one" should act is often determined by an image one has of how one such as oneself (say, a bank manager, lion tamer, prostitute, etc.) acts. This image usually corresponds to some sort of social norm, but this does not mean that all acting in accordance with social norms is inauthentic: The main point is the attitude one takes to one's own freedom and responsibility, and the extent to which one acts in accordance with this freedom.The Other and the LookThe Other (when written with a capital "O") is a concept more properly belonging to phenomenology and its account of intersubjectivity. However, the concept has seen widespread use in existentialist writings, and the conclusions drawn from it differ slightly from the phenomenological accounts. The experience of the Other is the experience of another free subject who inhabits the same world as a person does. In its most basic form, it is this experience of the Other that constitutes intersubjectivity and objectivity. To clarify, when one experiences someone else, and this Other person experiences the world (the same world that a person experiences), only from "over there", the world itself is constituted as objective in that it is something that is "there" as identical for both of the subjects; a person experiences the other person as experiencing the same things. This experience of the Other's look is what is termed the Look (sometimes the Gaze).

While this experience, in its basic phenomenological sense, constitutes the world as objective, and oneself as objectively existing subjectivity (one experiences oneself as seen in the Other's Look in precisely the same way that one experiences the Other as seen by him, as subjectivity), in existentialism, it also acts as a kind of limitation of freedom. This is because the Look tends to objectify what it sees. As such, when one experiences oneself in the Look, one doesn't experience oneself as nothing (no thing), but as something. Sartre's own example of a man peeping at someone through a keyhole can help clarify this: at first, this man is entirely caught up in the situation he is in; he is in a pre-reflexive state where his entire consciousness is directed at what goes on in the room. Suddenly, he hears a creaking floorboard behind him, and he becomes aware of himself as seen by the Other. He is thus filled with shame for he perceives himself as he would perceive someone else doing what he was doing, as a Peeping Tom. The Look is then co-constitutive of one's facticity.Another characteristic feature of the Look is that no Other really needs to have been there: It is quite possible that the creaking floorboard was nothing but the movement of an old house; the Look isn't some kind of mystical telepathic experience of the actual way the other sees one (there may also have been someone there, but he could have not noticed that the person was there). It is only one's perception of the way another might perceive him.